1. Field of the Invention
The invention relates to an electricity-generating installation on board a motor vehicle, of the type equipped with a fuel-cell stack.
The invention relates more particularly to an electricity-generating installation on board a motor vehicle, of the type equipped with a fuel-cell stack provided with at least one orifice for evacuation of residual gases, which are composed mainly of air and water vapor and which are discharged into an evacuation conduit in which there is disposed a condenser that liquefies the water vapor, and in which a compressor is interposed upstream from the condenser, the liquid water being diverted from the evacuation conduit to a liquid water circuit.
2. Description of Related Art
Fuel-cell stacks are used in particular to supply the electrical energy necessary for propulsion of motor vehicles. The fuel-cell stack is then mounted on board the vehicle.
A fuel-cell stack is composed mainly of two electrodes, an anode and a cathode, which are separated by an electrolyte. This type of stack permits direct conversion, to electrical energy, of the energy produced by the following oxidation-reduction reactions:
a reaction of oxidation of a fuel, which continuously feeds the anode; and
a reaction of reduction of an oxygen carrier, which continuously feeds the cathode.
The fuel-cell stacks used to supply electrical energy on board motor vehicles are generally of the solid-electrolyte type, especially with an electrolyte formed by a polymer membrane. Such a stack uses especially hydrogen (H2) and oxygen (O2) as the fuel and oxygen carrier respectively.
In contrast to combustion engines, which discharge a non-negligible quantity of polluting substances with the exhaust gases, the fuel-cell stack offers in particular the advantage of discharging mainly water, which is produced by the reduction reaction at the cathode.
The stack also discharges part of the oxygen carrier that has not reacted in the form of cathode evacuation gas, and it may also discharge part of the fuel that has not reacted in the form of anode evacuation gas. In the latter case, the fuel is generally burned before being discharged to the atmosphere in the form of water vapor.
In addition, the oxygen carrier of a stack of the type described in the foregoing can be ambient air, the oxygen (O2) of which undergoes reduction.
The oxygen carrier is generally humidified before being injected at the cathode, so that the membrane of polymer material is not damaged, for example by drying. This humidification operation is also applied to the fuel when the latter leaves the anode via an anode evacuation orifice.
The water necessary for humidification of the membrane is generally recovered at the stack outlet, and more particularly in the cathode evacuation gases, which contain water in liquid or vapor form, produced by the reaction of reduction of the oxygen carrier at the cathode.
Water recovery at the cathode outlet effectively has the advantage that there is no need for frequent replenishment of the water reserves of the vehicle. In addition, if sufficient water can be recovered to humidify the membrane, it is not necessary that the vehicle be equipped with a large-volume water reservoir.
To recover the water produced at the cathode, it is known that a condenser can be disposed in the stream of cathode evacuation gas. For optimal operation of the fuel-cell stack being fed with oxygen carrier and fuel under atmospheric pressure, this type of condenser generally requires a cooling source, whose temperature must be maintained between 20 and 30° C.
This solution is not usable, because motor vehicles are generally designed to operate in an environment whose temperature may vary between −20° C. and approximately 45° C. The use of a condenser therefore necessitates the use of a costly air-conditioning device, which is not available in all vehicle models.
It is known, therefore, to raise the oxygen-carrier pressure in the cathode circuit while retaining the condenser. By raising the pressure of the evacuation gases containing the water vapor, the dew point temperature of the water vapor is also effectively raised. The dew point temperature is the temperature at which the water vapor condenses. A condensation fog is then deposited on the surfaces whose temperature is lower than the dew point.
Thus, when the evacuation gases are injected into the condenser, at a pressure of 4 bar, for example, the cooling source of the condenser must then be maintained at a temperature of approximately 60° C. in order to function in optimum manner. It is much easier to maintain the cooling source of the condenser at a higher temperature than the ambient temperature.
However, such a solution requires that all feed circuits of the fuel-cell stack be kept under pressure, with the risk of degradation of the said stack. It is therefore necessary to use a non-negligible portion of the energy supplied by the stack to compress the oxygen-carrier and fuel circuits, to the detriment of the efficiency of the fuel-cell stack.